Turn your sound on, then press play.
A balloon rocket has no engine and no batteries. It carries its own air along for the ride, and that air does all the work.
When you blow up a balloon, you stretch the rubber. The stretched rubber squeezes the air inside, like a hand squeezing a sponge. The air is packed in tight and it wants out. When you let go of the neck, the rubber squeezes the air out the back.
Here is the big rule. When the balloon pushes air backward, the air pushes the balloon forward. Scientists call this action and reaction. The two pushes are equally strong, and they always go in opposite directions.
A scientist named Isaac Newton wrote this rule down more than 300 years ago. Real rockets use it too. They shoot hot gas down, and the gas pushes the rocket up. The balloon is not pushing off the chair or off the air behind it. A rocket even works in space, where there is nothing to push against.
Pick where the opening points. Then watch the air and the balloon.
The forward push on a rocket is called thrust. A balloon rocket has thrust for as long as air is coming out. When the air runs out, the thrust is gone. Then the rocket coasts and slows to a stop.
Let go of a balloon with no string and it zips around the room in loops. The string and the straw work like a train track. They keep the rocket pointed in one direction, so all of the thrust pushes it the same way.
A force is a push or a pull. Forces change how things move. They can start something, speed it up, slow it down, or stop it. On your balloon rocket, thrust pushes forward and other forces push back. Keep an eye out for which force is winning.
Friction is a force that happens when two things rub together. It always works against motion. Rub your hands together fast and you can feel friction as heat.
On a balloon rocket, the straw rubs on the string for the whole ride. That friction steals speed. Your job as an engineer is to make the track as slippery as you can.
The air in the balloon is the fuel for your rocket. More air means the thrust lasts longer, so the rocket can travel farther.
A bigger balloon is not free, though. A big balloon has to shove more air out of its way, and that means more drag. A balloon that is blown up too far can also pop. Fill it until it is big and round but still a little bit soft.
Count your breaths every time you fill the balloon. If you use three breaths on every run, every run starts with the same amount of fuel. That keeps your tests fair.
Every rocket has to carry its own weight. Yours carries the balloon, the straw, and the tape. Anything extra that a rocket carries is called cargo.
Extra weight hurts a balloon rocket in two ways. First, a heavier rocket is harder to speed up, so the same thrust gives it less speed. Second, a heavier rocket presses the straw down harder on the string, and that makes more friction.
Real rocket engineers fight this same problem. Every extra pound of cargo needs more fuel to move it. That is why real rockets are built to be as light as possible.
Engineers often have to give up a little of one thing to get more of another. That is called a trade-off. Air is a good example. More air gives you longer thrust, but it also gives you a bigger balloon with more drag. The best design is usually somewhere in the middle, and testing is how you find it.
Engineers test their ideas before they build. In this lab you launch a pretend balloon rocket down a string that is 30 feet long. Change the settings, launch it, and see how far it goes.
The fair test rule: change only one thing at a time. If you change two things at once, you cannot tell which one made the difference.
Cargo in this lab is paper clips taped to the straw.
No two launches are exactly alike, so the same rocket can stop in a slightly different spot each time. That is why engineers test more than once.
| Run | Air | String | Cargo | Distance | How it went |
|---|
No runs yet. Press the green button to launch.
The building steps open when every mission is finished. Here is what you still need to do:
You earned it. This is a job for two people, so find your partner. Follow each step in order and check the box when a step is done.
Put two chairs about 12 feet apart. That is about six big steps. Pick a spot where nobody needs to walk through.
Tie one end of the string to the top of the first chair. If knots are tricky, wrap the string around twice and tape it down. This is your start chair.
If your straw has a bendy part, cut it off. Push the loose end of the string all the way through the straw, like threading a bead.
Tie the loose end to the second chair at the same height as the first. Then scoot the second chair back until the string is tight, like a guitar string. A tight, level string is a fast track.
Push the straw along the string with one finger. It should glide easily from one chair to the other. If it catches, pull the string tighter or try a smoother string.
Tear off two pieces of tape, each about as long as your finger. Stick them lightly to the edge of the start chair. You will not have a free hand in a minute.
Blow up the balloon and count your breaths. Do not tie it. Pinch the neck shut with your fingers and keep pinching.
Slide the straw to the start chair. Hold the balloon under the straw with the opening pointing back at the start chair. Your partner tapes the balloon to the straw with both pieces of tape. Line the balloon up with the string so it is not crooked.
Check that the track is clear. Count down from three. Then let go of the neck. Do not push the balloon, because a push is not a fair test.
Measure from the start chair to the spot where the straw stopped. Write the distance in your run log below.
Remember the fair test rule. Change only one thing between runs, and write down what you changed.
| Run | What I changed | Feet |
|---|
| What happened | Why | Try this |
|---|---|---|
| It barely moves | Too much friction, or the string is saggy | Pull the string tighter. Try a smoother string. |
| It twists around the string | The balloon is taped on crooked | Tape it again, straight under the straw. |
| It stops halfway | Not enough air, or too much weight | Add a breath of air. Take off some cargo. |
| It goes the wrong way | The opening points at the far chair | Turn the balloon so the opening points back at the start chair. |
| The balloon falls off the straw | The tape let go | Use fresh tape and press it down firmly. |
Write three or more sentences. What did you change on your rocket or your track? What happened? Why do you think it happened? Use at least two of these words: thrust, friction, force, action, reaction, weight.
Big idea question: which force slowed your rocket down the most? How could you tell?
Back to all lessons | Straw Rocket Mission | Egg Drop Design Lab | Mr. Lehmkuhl